New Problem "Remove Invalid Parentheses"

README.md

@@ -9,6 +9,7 @@ LeetCode
 | # | Title | Solution | Difficulty |
 |---| ----- | -------- | ---------- |
 |303|[Range Sum Query - Immutable](https://leetcode.com/problems/range-sum-query-immutable/)  | [C++](./algorithms/cpp/rangeSumQuery-Immutable/rangeSumQuery-Immutable.cpp)|Easy|
+|301|[Remove Invalid Parentheses](https://leetcode.com/problems/remove-invalid-parentheses/) | [C++](./algorithms/cpp/removeInvalidParentheses/RemoveInvalidParentheses.cpp) |Hard|
 |300|[Longest Increasing Subsequence](https://leetcode.com/problems/longest-increasing-subsequence/)  | [C++](./algorithms/cpp/longestIncreasingSubsequence/longestIncreasingSubsequence.cpp)|Medium|
 |299|[Bulls and Cows](https://leetcode.com/problems/bulls-and-cows/)  | [C++](./algorithms/cpp/bullsAndCows/bullsAndCows.cpp)|Easy|
 |297|[Serialize and Deserialize Binary Tree](https://leetcode.com/problems/serialize-and-deserialize-binary-tree/)  | [C++](./algorithms/cpp/serializeAndDeserializeBinaryTree/SerializeAndDeserializeBinaryTree.cpp)|Medium|

algorithms/cpp/removeInvalidParentheses/RemoveInvalidParentheses.cpp

@@ -0,0 +1,112 @@
+// Source : https://leetcode.com/problems/remove-invalid-parentheses/
+// Author : Hao Chen
+// Date   : 2015-11-11
+
+/*************************************************************************************** 
+ *
+ * Remove the minimum number of invalid parentheses in order to make the input string 
+ * valid. Return all possible results.
+ * 
+ * Note: The input string may contain letters other than the parentheses ( and ). 
+ * 
+ * Examples:
+ * 
+ * "()())()" -> ["()()()", "(())()"]
+ * "(a)())()" -> ["(a)()()", "(a())()"]
+ * ")(" -> [""]
+ * 
+ * Credits:Special thanks to @hpplayer for adding this problem and creating all test 
+ * cases.
+ *               
+ ***************************************************************************************/
+
+#include <iostream>
+#include <vector>
+#include <string>
+#include <unordered_set>
+using namespace std;
+
+//DFS 
+void removeInvalidParenthesesHelper(string& s, int index, int pair,
+                                    int remove_left, int remove_right, 
+                                    string solution, unordered_set<string>& result) {
+
+    char ch = s[index];
+
+    //recusive ending 
+    if ( ch == '\0' ) {
+        if (pair==0 && remove_left==0 && remove_right==0 ) {
+            result.insert(solution);
+        }
+        return;
+    }
+    //other char, move to next one
+    if ( ch != '(' && ch != ')' ) {
+        removeInvalidParenthesesHelper(s, index+1, pair, remove_left, remove_right, solution+ch, result);
+        return;
+    }
+    
+    //if we meet left one, and we need remove left one, 
+    //then we have two choices : remove it, OR keep it.
+    if ( ch == '(' ) {
+        //revmoe it
+        if (remove_left > 0 ) { 
+            removeInvalidParenthesesHelper(s, index+1, pair, remove_left-1, remove_right, solution, result);
+        }
+        //keep it
+        removeInvalidParenthesesHelper(s, index+1, pair+1, remove_left, remove_right, solution+ch, result);
+    }
+    
+    //if we meet right one, and we need to remove right one,
+    //then we have two choices as well: remove it, or keep it if there are some left already.
+    if ( ch == ')' ) {
+        if (remove_right > 0 ) {
+            removeInvalidParenthesesHelper(s, index+1, pair, remove_left, remove_right-1, solution, result);
+        }
+        if (pair > 0){
+            removeInvalidParenthesesHelper(s, index+1, pair-1, remove_left, remove_right, solution+ch, result);
+        }
+    }
+
+
+}
+
+vector<string> removeInvalidParentheses(string s) {
+
+    //Calculating how many left/right parentheses need be removed!
+    int remove_left = 0, remove_right = 0;
+    for(auto c : s) {
+        if ( c == '(' ) {
+            remove_left++;
+        }else if ( c == ')' ){
+            if (remove_left > 0) {
+                remove_left--; // if we matched
+            }else{
+                remove_right++;
+            }
+        }
+    }
+
+    unordered_set<string> result;
+    removeInvalidParenthesesHelper(s, 0, 0, remove_left, remove_right, "", result);
+    return vector<string>( result.begin(), result.end() );
+}
+
+
+void printVector(vector<string> result) {
+    for( int i=0; i<result.size(); i++) {
+        cout << i << ") " << result[i] << endl;
+    }
+}
+
+
+int main(int argc, char** argv) {
+    string s = "()())()";
+    if (argc>1) {
+        s = argv[1];
+    }
+    cout << s << endl;
+    printVector( removeInvalidParentheses(s) );
+    
+    return 0;
+}